Delivery of tractive force to vehicle wheels must be accomplished as smoothly as possible. For example, shifting a vehicle between available speed ratios should be done without abrupt changes in torque that will be noticeable to vehicle occupants. Automatic transmissions typically rely on electronic and hydraulic controls to engage and disengage clutches and brakes to accomplish speed ratio changes. Although sufficient for their intended purpose, such controls may add complexity and cost.
Some transmissions have been proposed that use centripetal clutches with masses attached rotationally to the output member, so that as engine speed rises, so does the torque capacity of the clutch. Such transmissions are not able to passively downshift if relatively high output speed is constant, because the centripetal clutch will remain engaged.
Other transmissions have been proposed that use intermeshing helical gears of a planetary gear set that create an axial thrust when carrying torque. The axial thrust acts as a release force opposing engagement of a clutch that connects two members of the gear set to one another. However, once the clutch is engaged in such a transmission, the gear set is in a direct drive mode, not carrying torque, and thus not creating a release force. If input torque in such a transmission increases, the clutch will slip, possibly for an extended period of time, which could damage the clutch.